Norepinephrine, also called noradrenaline, is a catecholamine hormone and neurotransmitter involved in cardiovascular regulation, vascular tone control, alertness, stress adaptation, and sympathetic nervous system signaling. Unlike epinephrine, which strongly influences metabolic mobilization and bronchodilation, norepinephrine primarily regulates blood vessel constriction, blood pressure maintenance, and autonomic nervous system responsiveness.
The hormone increases peripheral vascular resistance by stimulating alpha-adrenergic receptors within vascular smooth muscle. It also contributes to attention, arousal, vigilance, and neural responsiveness within the central nervous system. During stress-related conditions, norepinephrine coordinates rapid cardiovascular and neurological adaptation by integrating autonomic signaling with endocrine communication pathways.
Norepinephrine is produced by sympathetic nerve terminals and chromaffin cells of the adrenal medulla. It is synthesized from the amino acid tyrosine through sequential enzymatic reactions involving tyrosine hydroxylase, aromatic amino acid decarboxylase, and dopamine beta-hydroxylase. In adrenal medullary tissue, norepinephrine may be further converted into epinephrine through phenylethanolamine N-methyltransferase activity.
Within sympathetic neurons, norepinephrine is packaged into vesicles and released at neuroeffector junctions where it acts locally on target tissues. Adrenal-derived norepinephrine also enters systemic circulation and exerts endocrine effects on cardiovascular and metabolic tissues.
Norepinephrine secretion is regulated primarily by sympathetic nervous system activation. Physical stress, emotional stimulation, hypoglycemia, exercise, orthostatic changes, cold exposure, and low blood pressure increase release. Sympathetic preganglionic neurons regulate adrenal medullary secretion through cholinergic signaling pathways.
The hormone acts mainly through alpha-adrenergic and beta-adrenergic receptors distributed throughout vascular tissue, heart, kidneys, adipose tissue, and nervous system structures. Intracellular signaling involves cyclic AMP pathways, calcium signaling systems, and phospholipase-mediated cascades. Reuptake transporters and monoamine oxidase pathways regulate clearance and termination of signaling. Through these integrated autonomic and endocrine mechanisms, norepinephrine maintains vascular responsiveness, cardiovascular stability, stress adaptation, and neural alertness during changing physiological demands.
Catecholamine from sympathetic neurons and adrenal medulla that raises vascular tone and supports acute energy mobilization.
